This invention relates to an apparatus for limiting a transient fault overcurrent which occurs upon a fault such as a ground fault or a line shortcircuit within a power system or between different power systems such as between AC power distribution lines or AC power transmission lines, and in particular to an AC current limiting apparatus (hereinafter called a current limiting apparatus) wherein upon the occurrence of the above faults, the acceleration of a generator is suppressed to improve a transient stability and to protect equipment within a power system, or a required breaking capacity of a circuit breaker and in turn the duty of power transmission or distribution equipment is reduced by limiting the current to be interrupted.
Tuned current limiting apparatuses are known where circuit systems are used as shown in FIGS. 1 and 2 which, for convenience's sake, respectively illustrate only one phase of a current limiting apparatus, generally designated by a reference numeral 10, and which is connected between interconnecting junctions a and b of two different kinds of power systems S1 and S2.
In FIG. 1, the current limiting apparatus 10 is formed of an air core or iron core type reactor 12, a capacitor 14 serially connected to the reactor 12, a closing device 16 connected in parallel with the capacitor 14, and a resistor 18 connected in series with the closing device 16 across the capacitor 14, where the closing device 16 comprises a gap, a mechanical or semiconductor device, or a non-linear resistive element. Japanese Pat. No. 987,109 issued on Feb. 21, 1980, which corresponds with the U.S. Pat. application Ser. No. 411,888 by John Rosa et al., particularly discloses the use of SCRs as a semiconductor type closing device.
In operation of the circuit in FIG. 1, normally the systems S1 and S2 are substantially at the same potential and so the terminal voltage across the capacitor 14 is at an extremely low level. Therefore the closing device 16 is disposed in its state to form a series resonance circuit, of the reactor 12 and the capacitor 14, which is tuned with the line frequency of the systems S1 and S2. On this occasion, the systems S1 and S2 are interconnected with null impedance of the current limiting apparatus 10. However, when a fault occurs such that a transient overcurrent flows between the systems S1 and S2, the terminal voltage across the capacitor 14 is rapidly increased, at which time, the capacitor 14 is discharged automatically if the closing device 16 is a gap, or by means of an external control if the closing device 16 is a mechanical or semiconductor switch, or through an electrical path formed by the automatic reduction to zero of impedance if the closing device 16 is a non-linear resistive element. This closing action of the closing device 16 electrically interconnects the systems S1 and S2 through the reactor 12 and the resistor 18 to suppress the overcurrent which flows through the systems S1 and S2 if the impedance of the reactor 12 is large as previously selected.
FIG. 2 shows another prior art current limiting apparatus, generally designated by reference numeral 10', comprising a bridged circuit arrangement using two sets of reactors 12 and 12A as well as capacitors 14 and 14A across bridging junctions c and d between which the closing device 16 and the resistor 18 are serially inserted, while the circuit of FIG. 1 is arranged with the combination of one set of the reactor 12 and the capacitor 14.
In operation of the circuit in FIG. 2, normally the systems S1 and S2 are substantially at the same potential and so the potential difference between the bridging junctions c and d is low. Therefore, the closing device 16 is disposed in its open state to form two sets of series resonance circuits consisting of the reactor 12 and the capacitor 14 as well as the reactor 12A and the capacitor 14A, each of which is tuned with the line frequency of the systems S1 and S2. In this case, both of these two sets of series resonance circuits have approximately zero impedance for directly interconnecting the systems S1 and S2. When a fault occurs such that a transient overcurrent flows through the systems S1 and S2, the potential difference between the bridging junctions c and d is rapidly increased causing the closing device 16 to be actuated, which bridges the junctions c and d, which forms two sets of parallel resonance circuits consisting of the reactor 12 and the capacitor 14A as well as the reactor 12A and the capacitor 14, whereby the systems S1 and S2 are interconnected via a large impedance (the impedance will be infinite if the combined resistance of the closing device 16 and the resistor 18 is zero), so that the overcurrent which flows between the systems S1 and S2 is suppressed.
Although it is advantageous that such a conventional series resonance type current limiting apparatus as in FIG. 1 has a simple arrangement and can be made cheaply, it has the following characteristic defects. Namely, first of all, since the limited current is inductive or lags in phase behind the voltage so that the restriking voltage upon the circuit interruption becomes high, a restriking voltage suppressing device or a circuit breaker with a good withstanding restriking voltage characteristics is additionally required. Secondly, the large inductance which is previously provided for the reactor 12 for enhancing the current limiting effects will raise the terminal voltage across the reactor 12 upon normal or fault occasions and so that the dielectric strength of the reactor 12 will have to be increased. Also, the addition of a restriking voltage suppressing device or a circuit breaker with a much better withstanding restriking voltage characteristics will be required. Thirdly, if the closing device 16 is composed of a non-linear resistive element, a discharging resistor or reactor etc will have to be provided since the capacitor 14 remain charged.
Further in such a series-parallel resonance type apparatus as in FIG. 2, since it has an opposite feature to the aforementioned series resonance type apparatus and the limited current is substantially resistive in phase with voltage, it is advantageous in that the transient stability of the systems is improved and the breaking duty for a circuit breaker is lightened because of the low restriking voltage upon the circuit interruption. However, the circuit arrangement is complicated and therefore expensive, compared to the series resonance type apparatus in FIG. 1. Moreover, although there is no problem when the inductance values of the reactors 12 and 12A are identical and the capacitance values of the capacitors 14 and 14A are also identical, the presence of component variations, if the variations are considerable, will cause a high circulating current to flow through the current limiting apparatus, and will disturb the resonance conditions. Finally, if the closing device 16 is composed of a non-linear resistive element, the capacitors 14 and 14A remain charged in this case either, so that a discharging resistor or reactor etc will disadvantageously have to be provided.
It is accordingly an object of the invention to provide a current limiting apparatus without the defects of the prior art as set forth above.